Abstract 5129: Subcellular MLH1 protein as first in class biomarker for CDK4/6 response in endocrine resistant ER+/HER2- breast cancer

Abstract Over 175, 000 diagnoses of estrogen receptor-positive breast cancer are made every year in the US. Of these, 40, 000 patients will go on to die of their disease. We recently identified defects in a principal mismatch repair (MMR) gene, MLH1, as a driver of resistance to standard-care treatment and poor patient outcomes. Moreover, we demonstrated increased sensitivity to palbociclib, a CDK4/6 inhibitor, in experimental model systems in vitro and in vivo and in clinical trial data. MLH1 is mutated in 5-10% of treatment-resistant tumors, i. e. , 4, 000 patient tumors every year. However, most of these mutations are missense and variants of unknown significance (VUS). The impact of these mutations on mismatch repair capability and on treatment resistance remains uncertain. Here, we use experimental model systems to phenotype MLH1 VUS´ in vitro and in vivo using xenograft systems. Specifically, we use nuclear localization of MLH1 and repair of mismatches through a mismatch repair deficiency assay to test for mismatch repair capability in ER+ breast cancer cells engineered to express mutations in MLH1 that are found in patient tumors associated with poor survival. Second, we assess the ability of variants of MLH1 to induce activation of G1/S cell cycle checkpoints by activating Chk2. Lastly, we directly test the ability of variants in MLH1 to induce resistance to standard-care endocrine therapies and sensitivity to palbociclib, using 2D and 3D growth assays and xenograft experiments. Results from these experiments serve to systematically catalog VUS´ in MLH1 based on their predictive potential. We identified for the first-time specific mutation in MLH1 that induces resistance to endocrine therapy by stalling nuclear translocation of MLH1 which inhibits Chk2 activation. Cells with impaired Chk2 activation maintain a high CDK4/6 levels and are therefore more sensitive to CDK4/6 inhibition. Currently, there are few tests available in the clinic to predict response to therapies early in the timeline of ER+ breast cancer patients. Due to the nature of the disease, this can result in years of overtreatment. Overtreatment comes at great financial, physical, and emotional cost to patients and their families. Simultaneously, a lack of predictive markers can result in missed opportunities of presenting patients likely to relapse on standard care with personalized alternative therapeutic strategies. Based on our findings, cytoplasmic MLH1 may serve as first in-class biomarker for CDK4/6 response in ER+/HER2- patients. Citation Format: Aloran Mazumder, Elena Oropeza, Nindo Punturi, Svasti Haricharan. Subcellular MLH1 protein as first in class biomarker for CDK4/6 response in endocrine resistant ER+/HER2- breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5129.